Vacuum Pump

ABSTRACT

A vacuum pumping arrangement is described for evacuating a load lock chamber. A booster pump comprises a molecular drag stage and a multi-stage centrifugal compressor mechanism. A backing pump comprises a multi-stage centrifugal compressor mechanism exhausting pumped fluid at atmospheric pressure. Such arrangements can reduce noise, size and vibration levels associated with conventional load lock pumping arrangements.

The invention relates to a vacuum pump.

Vacuum processing is commonly used in the manufacture of semiconductordevices to deposit thin films on to substrates. Typically, a processingchamber is evacuated using a vacuum pump to a very low pressure, which,depending on the type of process, may be as low as 10⁻⁶ mbar, and feedgases are introduced to the evacuated chamber to cause the desiredmaterial to be deposited on one or more substrates located in thechamber. Upon completion of the deposition, the substrate is removedfrom the chamber and another substrate is inserted for repetition of thedeposition process.

Significant vacuum pumping time is required to evacuate the processingchamber to the required pressure. Therefore, in order to maintain thepressure in the chamber at or around the required level when changingsubstrates, transfer chambers and load lock chambers are typically used.The capacity of the load lock chamber can range from just a few litresto several thousand litres for some of the larger flat panel displaytools.

The load lock chamber typically has a first window, which can beselectively opened to allow substrates to be transferred between theload lock chamber and the transfer chamber, and a second window, whichcan be selectively opened to the atmosphere to allow substrates to beinserted into and removed from the load lock-chamber. In use, theprocessing chamber is maintained at the desired vacuum by the processingchamber vacuum pump. With the first window closed, the second window isopened to the atmosphere to allow the substrate to be inserted into theload lock chamber. The second window is then closed, and, using a loadlock vacuum pump, the load lock chamber is evacuated until the load lockchamber is at substantially the same pressure as the transfer chamber,typically around 0.1 mbar. The first window is then opened to allow thesubstrate to be transferred to the transfer chamber. The transferchamber is then evacuated to a pressure at substantially the samepressure as the processing chamber, whereupon the substrate istransferred to the processing chamber.

When vacuum processing has been completed, the processed substrate istransferred back to the load lock chamber. With the first window closedto maintain the vacuum in the transfer chamber, the pressure in the loadlock chamber is brought up to atmospheric pressure by allowing anon-reactive gas, such as air or nitrogen, to flow into the load lockchamber. When the pressure in the load lock chamber is at or nearatmospheric pressure, the second window is opened to allow the processedsubstrate to be removed. Thus, for a load lock chamber, a repeatingcycle of evacuation from atmosphere to a medium vacuum (around 0.1 mbar)is required.

Load lock pumps are typically oil-free in their vacuum chambers, as anylubricants present in the vacuum chambers might cause contamination ofthe clean environment in which the vacuum processing is performed. Forexample, the “iH” series of BOC Edwards “dry” vacuum pumps comprise adry backing, or roughing, pump in combination with a single stage Rootsmechanism booster, or blower, pump mounted above the dry pump. Backingpumps are commonly multi-stage positive displacement pumps employinginter-meshing rotors. The rotors may have the same type of profile ineach stage or the profile may change from stage to stage.

For the larger flat panel display tools, the pumping speed of the loadlock pumps needs to be high, for example, up to 2000 m³/hour. Whilst aload lock pump formed from a dry backing pump using Roots and Northeymechanisms, in series with a Roots booster pump can provide such apumping speed, the relatively large foot-print of the pump combination,together with the level of noise and vibration generated during use,typically lead to the load lock pump being located remote from theprocessing tool, for example, in a basement. As well as beinginconvenient to the user, relatively long runs of large diameter pipework are needed to connect the load lock pump to the load lock chamber,significantly increasing installation costs.

It is an aim of at least the preferred embodiments of the presentinvention to solve these and other problems.

In summary, in accordance with the present invention at least one of thebooster pump and the backing pump in the conventional pumpingarrangement is replaced by a vacuum pump comprising a multi-stagecentrifugal compressor system. In one embodiment, both the booster pumpand the backing pump are replaced by a single vacuum pump exhausting toatmosphere. In a second embodiment, the booster pump is provided by asimilar vacuum pump to the first embodiment, having a reduced number ofcompressor stages, backed by a backing pump. This backing pump may be aconventional backing pump, or, in accordance with a third embodiment,may be a vacuum pump comprising a multi-stage centrifugal compressorsystem exhausting to atmosphere. Such a backing pump may be providedwith a conventional Roots booster pump. Thus, in one aspect, the presentinvention provides a vacuum pump comprising a multi-stage centrifugalcompressor mechanism for receiving fluid to be pumped and exhaustingpumped fluid substantially at atmospheric pressure.

Due to the reduced levels of size, noise and vibration associated with acentrifugal compressor system in comparison to the conventional drypumps, replacing one or both of the conventional backing and boosterpumps with a pump comprising a multi-stage centrifugal compressormechanism can enable at least part of the pumping arrangement to bemounted on the processing tool, thereby potentially avoiding theexpensive long runs of large diameter pipe work.

It is desirable to perform the evacuation of a vacuum chamber, such as aload lock chamber, from atmospheric pressure to a low pressure asquickly as possible. The faster that this evacuation can beaccomplished, the higher the rate of processing substrates becomes.However, during the initial stages of the evacuation of a chamber fromatmospheric pressure using a pump having a multi-stage pumpingmechanism, the compression of fluid by the pumping mechanism can causethe fluid pressure to increase above atmospheric pressure. This canresult in undesirable overloading of the exhaust stages of the pumpingmechanism. If such a pump is operated for a significant period in thiscondition, damage can occur in the form of seals and/or bearingsfailing, or by impact between the fragile rotating impellers and pump'shousing.

In view of this, in another aspect the present invention provides amulti-stage centrifugal compressor mechanism comprising a housing, adrive shaft rotatably mounted within the housing, a plurality of fixedmembers disposed within the housing and defining a plurality ofinterconnected fluid chambers, a plurality of impellers mounted on thedrive shaft and disposed relative to the fixed members such that eachimpeller delivers compressed fluid to a respective fluid chamber, a isbypass channel extending between two of the fluid chambers to enablefluid to pass between those chambers without compression, and means forcontrolling the flow of fluid through the bypass channel. Compressedfluid can thus be conveyed between fluid chambers without compression,which can enable a larger upstream pumping stage to operate at fullspeed without causing the pumped fluid to be pressurised aboveatmospheric pressure.

The control means is thus preferably arranged to open the bypass channelunder the influence of a pressure difference between said two of thefluid chambers, and in particular when the pressure in an upstream oneof said two of the fluid chambers is greater than the pressure in adownstream one of said two of the fluid chambers.

In a preferred embodiment, said two of the fluid chambers are adjacentfluid chambers of the compressor mechanism, although one or more otherfluid chambers may, alternatively, separate these two fluid chambers.For example, one of the fluid chambers may be the first, lowest pressurefluid chamber of the pumping mechanism, and the other fluid chamber bythe last, highest pressure fluid chamber of the pumping mechanism. Wherethese two fluid chambers are adjacent, however, the bypass channel mayconveniently pass through the fixed member located between the fluidchambers.

The control means preferably comprises valve means, for example, a valvemember displaceable in use between a closed position and an openposition by pressurised fluid. Such a valve member may be convenientlyprovided by a flap valve, which can be conveniently positioned within afluid chamber to control the flow of fluid into that fluid chamber fromthe bypass channel.

Preferably, the mechanism comprises, for each fluid chamber, arespective bypass channel extending between that fluid chamber and theadjacent downstream fluid chamber, and means for controlling the flow offluid through each bypass channel.

Centrifugal compressor mechanisms are susceptible to surging of pumpedfluid when the specific flow rate of the pumped fluid through a stage ofthe compressor mechanism is relatively low. The surging manifests itselfin a backflow of fluid into the compressor impeller, and adverselyaffects the efficient operation of the vacuum pump, and in extremeconditions, may actually damage the pump. In view of this, the mechanismpreferably comprises surge control means for controlling surge withinthe compressor mechanism. Therefore, in a further aspect the presentinvention provides a multi-stage centrifugal compressor mechanismcomprising a housing, a drive shaft rotatably mounted within thehousing, a plurality of fixed members disposed within the housing anddefining a plurality of interconnected fluid chambers, a plurality ofimpellers mounted on the drive shaft and disposed relative to the fixedmembers such that each impeller delivers compressed fluid to arespective fluid chamber, and surge control means for controlling surgewithin the multi-stage centrifugal compressor mechanism.

The surge control means preferably comprises means for conveying astream of fluid to each fluid chamber, and means for controlling therate of flow of the fluid stream into each fluid chamber. In oneembodiment, the conveying means is arranged to convey a stream of gas,such as air, nitrogen or an inert gas, to each fluid chamber. In anotherembodiment, the conveying means is arranged to convey a stream ofcompressed fluid to each fluid chamber. In either case, the rate of flowthrough the compressor mechanism can be maintained at a value above thatat which surging will occur.

Where the conveying means is arranged to convey a stream of compressedfluid to each fluid chamber from a downstream fluid chamber, theconveying means preferably comprises, for each fluid chamber, a fluidpassage (separate from the previously-mentioned bypass channel)extending between that fluid chamber and the adjacent downstream fluidchamber. These fluid passages are preferably co-axial.

The means for controlling the rate of flow of the fluid stream into eachfluid chamber preferably comprises valve means. The valve means maycomprise a series of valves for controlling fluid flow throughrespective fluid passages or a spool valve for controlling fluid flowthrough each fluid passage. The valve means is preferably located atleast partially within the chamber, thereby avoiding the need to provideexternal pipe connections. The valve means may be controlled by aseparate controller. In order to control the valve means, a pressuresensor may be provided to monitor the pressure of fluid passing througha pump inlet, a signal from the inlet sensor being supplied to a controlsystem which controls the opening and closing of the valve means. Inaddition, or alternatively, pressure sensors may be provided within thepumping mechanism to monitor pressure fluctuation within the pumpingmechanism, and thus detect the onset of surging.

Each impeller preferably has on one side thereof a plurality of vanes orblades extending between the inner periphery and the outer peripherythereof. Each blade preferably follows a curved path. To facilitatemanufacture, each fixed member preferably comprises a disc integral witha respective part of the housing.

Fluid that is compressed by the compressor mechanism typically becomeshot. In order to cool fluid pumped by the compressor mechanism,particularly at the exhaust stages, the mechanism preferably comprisesmeans for cooling each fixed member. For example, a plurality of coolingfins may be provided on one side thereof. Alternatively, or in addition,the cooling means may comprise means for supplying a flow of coolant toeach fixed member. This can provide direct cooling of both the coolingfins (where provided) and the fixed plate. The cooling fins may belocated between the fixed plate and a diffuser plate for directing astream of compressed fluid from an impeller to a fluid chamber so thatthe fins can also provide for cooling of the diffuser plate.

The present invention also provides a vacuum pump comprising acompressor mechanism as aforementioned.

Excessive heating of the compressor mechanism may occur if the pump isoperated over a relatively long period at a relatively high pressure,for example, if a door to a load lock chamber evacuated by the pump hasbeen inadvertently left open. In order to prevent excessive heating ofthe pump, the temperature of the pump may be monitored, and the speed ofrotation of the compressor mechanism varied in response to the monitoredtemperature. This can enable the speed of the pump to be reduced in theevent of overheating, thereby reducing the temperature within the pump,and preventing the pump from being unduly operated at a high speed for arelatively long period.

Therefore, the pump preferably comprises means for monitoring thetemperature of the pump, and means for controlling the speed of rotationof the shaft in dependence on the monitored temperature. The monitoringmeans may be conveniently provided by any suitable temperature sensor,such as a thermocouple, located within or in close proximity to thehousing. A controller for controlling a motor driving the drive shaftmay provide the control means.

In order to cool the housing, to which heat will be transferred by thepumped fluid, an external cooling system may also be provided, forexample, in the form of a cooling jacket extending about at least partof the compressor mechanism.

Where the pump is to be used as a backing pump, the backing pump mayconsist of such a multi-stage centrifugal compressor mechanism, incombination with any suitable booster pump. Such a booster pump may beprovided by a pump comprising such a multi-stage centrifugal compressormechanism downstream from a molecular drag mechanism, the number ofstages of the compressor mechanism (for example, two) being smaller inthe booster pump than in the backing pump (for example, six or seven).Alternatively, the conventional combination of booster and backing pumpsmay be replaced by a single pump, this pump comprising a multi-stage(for example, six or seven stage) centrifugal compressor mechanismdownstream from a multi-stage (for example, four stage), molecular dragmechanism. The molecular drag mechanism preferably comprises amulti-stage Holweck mechanism having a plurality of channels arranged asa plurality of helixes. The drag stages may be arranged in series, inparallel for maximum pumped volume, or in a combination of both. Inorder to minimise the length of the pump the molecular drag mechanismpreferably at least partially surrounds a motor for rotating the driveshaft. For instance, where the molecular drag pumping mechanism is aHolweck mechanism, a rotor element of the molecular drag pumpingmechanism typically comprises a cylinder mounted for rotary movementwith the rotor elements of the compressor mechanism, which cylinder mayat least partially surround the motor. This, in a further aspect thepresent invention provides a vacuum pump comprising a multi-stagecentrifugal compressor mechanism comprising a plurality of rotorelements mounted on a rotatably mounted drive shaft, and, upstreamtherefrom, a molecular drag mechanism comprising at least one rotorelement mounted on the drive shaft, wherein the at least one rotorelement of the molecular drag mechanism at least partially surrounds amotor for rotating the drive shaft.

As discussed above, for rapid pump down of a chamber the pump may beprovided with valve means for enabling compressed fluid to by-pass oneor more of the impellers of the multi-stage centrifugal compressormechanism, allowing the pump to pump down at full inlet speed even whenthe exhaust stages of the compressor mechanism are somewhat smaller thanthe inlet stages. With such a design, the backing pump may become arestriction to the flow of fluid through the pumping arrangement.Therefore, in a preferred arrangement a fluid by-pass conduit isconnected between an exhaust from the booster pump and an exhaust fromthe backing pump, with means being provided for controlling the flow offluid through the by-pass. Such an arrangement may be provided for anycombination of booster and backing pumps.

Preferred features of the present invention will now be described, byway of example only, with reference to the accompanying drawings, inwhich:

FIG. 1 is a cross-section through a first embodiment of a vacuum pump;

FIG. 2 is a cross-section through a second embodiment of a vacuum pump,which is similar to that of FIG. 1 with a different surge controlmechanism;

FIG. 3 is a cross-section through an embodiment of a booster pump, whichis similar to that of FIG. 1 with a reduced number of compressor stages;

FIG. 4 is an enlarged view of part of the cross-section of FIG. 3;

FIG. 5 is a cross-section through an embodiment of a backing pump, whichis similar to that of FIG. 1 without a drag mechanism;

FIG. 6 illustrates schematically an arrangement of valves in a pumpingarrangement comprising a booster pump in series with a backing pump; and

FIG. 7 illustrates schematically an arrangement for controlling thespeed of a booster pump.

With reference to FIG. 1, a vacuum pump 10 suitable for evacuating aload lock chamber comprises a housing 12 having three parts 14, 16, and18. An inlet 20 for the pump 10 is located in the first part 14 of thehousing 12, and an exhaust 21 for the pump 10 is located in the thirdpart 18 of the housing 12.

The first part 14 of the housing 12 houses a multi-stage molecular dragpumping mechanism 22. As illustrated in FIG. 1, in this embodiment themolecular drag pumping mechanism is provided by a four-stage Holweckmechanism 22, although any suitable number of pumping stages may beprovided. The rotor of the Holweck mechanism 22 comprises twocarbon-fibre cylinders 24, 26, mounted concentrically on a disc-likeimpeller 28 integral with or, as illustrated, mounted on is a rotatableshaft 30. The shaft 30 is supported at each end by lubricant freebearings (not shown), preferably magnetic bearings, and is driven by amotor 31 housed by the third part 18 of the housing 12.

Each cylinder 24, 26 of the Holweck mechanism 22 has smooth inner andouter surfaces. The stator of the Holweck mechanism comprises aplurality of cylinders 32, 34 and 36 concentrically arranged with andsurrounding the rotor cylinders 24, 26, the outermost cylinder 36 beingprovided by the first part 14 of the housing 12. Helical grooves areformed on the outer surfaces of the innermost stator cylinder 32, theinner and outer surfaces of the middle stator cylinder 34 and the innersurface of the outermost stator cylinder 36 to define co-axial helicalfluid channels 38, 40, 42, 44, which receive fluid from the pump inlet20 and exhaust pumped, compressed fluid to a common exhaust port 48through openings 50 formed in the disc-like impeller 28.

The second part 16 of the housing 12 may be conveniently provided by aplurality of co-axial rings 16 a, 16 b, 16 c, 16 d, 16 e, and 16 f, andhouses a multi-stage centrifugal compressor mechanism 52. In theembodiment shown in FIG. 1, the compressor mechanism 52 comprises sevenpumping stages. Each of the first six pumping stages comprises arespective fluid chamber 58, each defined between respective discs 60co-axially mounted on the inner wall of the second part 16 of thehousing 12. Apertures 62 in the discs 60 interconnect the fluid chambers58 to enable fluid to be conveyed from the exhaust port 48 of theHolweck mechanism 22 to the exhaust 21 of the pump through each of thefluid chambers 58 in turn.

Each fluid chamber 58 includes a rotor in the form of an impeller 54mounted on the shaft 30. Each impeller 54 has a plurality of curvedblades or vanes 56 located on the upper surface (as shown in FIG. 1) ofthe impeller 54. The impellers 54 are disposed relative to the discs 60such that, during use, each impeller 54 delivers compressed fluid to arespective fluid chamber 58.

Each fluid chamber 58 also includes a disc-like diffuser plate 64, eachintegral with a respective ring 16 a, 16 b, 16 c, 16 d, 16 e, and 16 ffor directing compressed fluid output from the each of the impellers 54radially outwardly. As a result, compressed fluid flows within the fluidchambers 58 in a serpentine manner; within each fluid chamber 58,compressed fluid initially flows radially outwards between the uppersurface (as shown) of the diffuser plate 64 and the facing, lowersurface of the upper disc 60 defining that fluid chamber, andsubsequently flows radially inwards between the lower surface (as shown)of the diffuser plate 64 and the facing, upper surface of the lower discdefining that fluid chamber.

Each of the diffuser plates 64 comprises a plurality of cooling fins 66,provided on the lower surface thereof, for cooling the compressed fluid.In order to cool the housing 12, to which heat will be transferred bythe fluid pumped by the compressor mechanism 52, an external pumpcooling system (not shown) may also be provided, for example, in theform of a cooling jacket extending about at least the second part 16 ofthe housing 12.

In use, the motor is activated to rotate the shaft 30 at a high speed,typically in the range from 15,000 to 80,000 rpm. Fluid enters the pump10 through the inlet 20, and passes in turn through the Holweckmechanism 22 and compressor mechanism 52 before being exhausted from theoutlet of the pump 10 at a pressure at or around atmospheric pressure.With the arrangement shown in FIG. 1, a pressure less than 1 mbar,typically at or around 0.1 mbar, can be generated in a load lock chamberconnected to the inlet 20 of the pump 10.

In order to inhibit surging within the compressor mechanism 52 atrelatively low flow rates, the pump 10 is provided with a surge controlmechanism for selectively increasing the rate of flow of fluid throughone or more of the pumping stages of the compressor mechanism 52. In thefirst embodiment shown in FIG. 1, a fluid port 68 is provided in each ofthe rings 16 a, 16 b, 16 c, 16 d, 16 e, 16 f, each port 68 extendingradially through the ring to allow a stream of fluid to be injected intothe fluid chamber 58. This stream of fluid may be provided by anysuitable source. In a first example, the fluid ports 68 of adjacentpumping stages may be connected via an arrangement of conduits locatedto one side of the pump 10, the conduits containing one or more valvesfor selectively opening the conduits to allow pumped fluid from onepumping stage to flow from the fluid port 68 of that stage to the fluidport 68 of the adjacent upstream pumping stage, thereby increasing therate of flow of fluid through the inlet to the pumping stage. In asecond example, a stream of purge gas, such as nitrogen or air, may beselectively supplied from a suitable source to one of more of the fluidports 68 in order to increase the rate of flow of fluid through one ormore of the pumping stages.

In a third example, as illustrated in FIG. 2, a passage 70 forcompressed fluid may be provided through the pumping stages of thecompressor mechanism 52 in addition to, or as an alternative to,providing fluid ports 68. The passage 70 is defined by a series ofco-axial apertures 72 formed in the discs 60 co-axially mounted on theinner wall of the second part 16 of the housing 12. A spool valve 74 isprovided for selectively opening and closing the apertures 72 to controlthe flow of compressed fluid through the passage 70. As illustrated, thespool valve 74 may be shaped so that movement of the valve 74 causes allof the apertures 72 to be opened simultaneously to allow compressedfluid to flow through the apertures 72 to adjacent upstream pumpingstages. Alternatively, the spool valve 74 may be shaped so that movementof the valve 74 causes each of the apertures 74 to be opened in turn,starting, for example, with the aperture 74 connecting the exhaustpumping stages of the compressor mechanism 52.

In order to control the valves in any of these three examples, apressure sensor may be provided to monitor the pressure of fluid passingthrough the pump inlet 20. A signal from the inlet sensor may besupplied to a control system, which controls the opening and closing ofthe, or each, valve in order to inhibit surging. In addition, oralternatively, pressure sensors may be provided within the pump 10 tomonitor pressure fluctuation within the pump, and thus detect the onsetof 15 surging. Motor current may also be used to indicate shaft torqueand power, and thus an estimation of inlet pressure.

As the pumps illustrated in FIGS. 1 and 2 exhaust fluid at or aroundatmospheric pressure, each pump 10 would be suitable for replacing boththe conventional booster and backing pump used for evacuating a loadlock chamber. Due to the reduced size of the pump 10 relative to thesize of the conventional combination of booster and backing pumps, anddue to the reduced noise and vibration levels associated with the pump10, the pump 10 may be conveniently mounted on the side of theprocessing tool.

By reducing the number of stages of the compressor mechanism 52, thepump 10 can be suitable for use as a booster pump. FIGS. 3 and 4illustrate an embodiment of such a booster pump 100. The booster pumpcomprises a housing having three parts 102, 104, and 106. An inlet 110for the pump 100 is located in the first part 102 of the housing, and anexhaust 112 for the pump 100 is located in the third part 106 of thehousing.

The second part 104 of the housing houses a multi-stage molecular dragpumping mechanism 120. As illustrated in FIG. 3, similar to the pump 10of FIG. 1 the molecular drag pumping mechanism is provided by afour-stage Holweck mechanism 120, although any suitable number ofpumping stages may be provided. The rotor of the Holweck mechanism 120comprises three carbon-fibre cylinders 122, 124, 126, mountedconcentrically on a disc-like impeller 128 integral with or, asillustrated, mounted on a rotatable drive shaft 130. The shaft 130 issupported at each end by rolling bearings 132 and is driven by a motor134 partially surrounded by the cylinders 122, 124, 126 of the Holweckmechanism 120.

Each cylinder of the Holweck mechanism 120 has smooth inner and outersurfaces. The stator of the Holweck mechanism 120 comprises a pluralityof cylinders 136, 138 and 140 concentrically arranged with andsurrounding the rotor cylinders 122, 124, 126, the outermost cylinder 36being provided by, or, as illustrated, mounted on the second part 104 ofthe housing. Helical grooves are formed on the outer surface of theinnermost stator cylinder 136, the inner and outer surfaces of themiddle stator cylinder 138 and the inner surface of the outermost statorcylinder 140 to define co-axial helical fluid channels which receivefluid from the pump inlet 110 through one or more openings 142 formed inthe disc-like impeller 128 and exhaust pumped, compressed fluid to acommon exhaust port 144.

The second part 106 of the housing may be conveniently provided by aplurality of co-axial rings 106 a, 106 b, 106 c, and 106 d, and houses amulti-stage centrifugal compressor mechanism 150. In the embodimentshown in FIGS. 3 and 4, the compressor mechanism 150 comprises fourpumping stages. Each of the first three pumping stages comprises arespective fluid chamber 158, each defined between respective discs 160co-axially mounted on the inner wall of the second part 106. Apertures162 in the discs 160 interconnect the fluid chambers 158 to enable fluidto be conveyed from the exhaust port 144 of the Holweck mechanism to theexhaust 112 of the pump through each of the fluid chambers 158 in turn.

Each fluid chamber 158 includes a rotor in the form of an impeller 154mounted on the shaft 130. Each impeller 54 has a plurality of curvedblades or vanes 156 located on the upper surface (as shown in FIG. 1) ofthe impeller 154. The impellers 154 are disposed relative to the discs160 such that, during use, each impeller 154 delivers compressed fluidto a respective fluid chamber 158.

Each fluid chamber 158 also includes a disc-like diffuser plate 164,each integral with a respective ring 106 a, 106 b, 106 c, 106 d, fordirecting compressed fluid output from the each of the impellers 154radially outwardly. As a result, compressed fluid flows within the fluidchambers 158 in a serpentine manner; within each fluid chamber 158,compressed fluid initially flows radially outwards between the uppersurface (as shown) of the diffuser plate 164 and the facing, lowersurface of the upper disc 160 defining that fluid chamber, andsubsequently flows radially inwards between the lower surface (as shown)of the diffuser plate 164 and the facing, upper surface of the lowerdisc 160 defining that fluid chamber.

Each of the diffuser plates 164 may comprise a plurality of cooling fins(not shown), provided on the lower surface thereof, for cooling thecompressed fluid. In order to cool the fins, a coolant may be conveyedthrough cooling channels defined between the lower (as shown) surface ofthe diffuser plate 164 and the facing, upper surface of the lower disc160.

In use, the motor is activated to rotate the shaft 130 at a high speed,typically in the range from 15,000 to 80,000 rpm. Fluid enters the pump100 through the inlet 110, and passes in turn through the Holweckmechanism 120 and compressor mechanism 150 before being exhausted fromthe outlet 112 of the pump 100 at a sub-atmospheric pressure.

Similar to the pump described in FIG. 1, a surge control mechanism maybe provided to inhibit surging within the compressor mechanism. Forexample, as shown in FIG. 4, a fluid port 168 may provided in each ofthe rings 106 a, 106 b, and 106 c, each port 168 extending radiallythrough the ring to allow a stream of fluid to be injected into arespective fluid chamber 158. This stream of fluid may be provided byany suitable source. Preferably, such a mechanism would be operated onlyat relatively low inlet pressures in order to maximise throughput atrelatively high inlet pressures.

In addition to such a surge control mechanism, an additional mechanismmay also be provided to enable rapid pump down of a chamber attached tothe inlet 110 of the booster pump 100 without overloading the exhauststages of the compressor mechanism. As shown in FIG. 4, one or more ofthe discs 160 are provided with bypass channels 170 for enablingcompressed fluid to pass to an adjacent, downstream fluid chamberwithout compression by an impeller 154. The channels 170 are normallyclosed by a valve mechanism 172, which in this embodiment is in the formof a pair of flap valves having a common mounting within the downstreamfluid chamber 158. The valve mechanism 172 is selectively opened by apressure differential between fluid within the adjacent fluid chambers158, so that when the pressure of fluid in the upstream fluid chamber isgreater than that in the downstream fluid chamber, the valve opens toenable fluid to pass from the upstream fluid chamber to the downstreamfluid chamber without compression.

This can enable gas being pumped by the pump 100 to pass through one ormore of the smaller, exhaust stages of the compressor mechanism 150without compression, thereby avoiding the gas from being compressedabove atmospheric pressure by those exhaust stages and thus preventingthose stages from becoming overloaded. The booster pump 100 may be usedin combination with any suitable backing pump. FIG. 5 illustrates anembodiment of a backing pump 200 employing a multi-stage centrifugalcompressor mechanism, which would be suitable for use with such abooster pump, or any conventional booster pump. The backing pump 200 issimilar to the pump 10 illustrated in FIG. 1, with the exception thatthe backing pump 200 does not require a drag mechanism as the fluidentering the backing pump 200 would be at a higher pressure than thatentering the pump 10. In other words, the backing pump 200 comprises amulti-stage compressor mechanism 252 for receiving fluid from the pumpinlet 220 and exhausting pumped fluid at or around atmospheric pressurefrom pump outlet 221. The compressor mechanism 252 of the backing pump200 is similar to the compressor mechanism 52 of the pump 10, and so isnot described in further detail here.

During pump down of a chamber attached to a series combination of thebooster pump 100 and backing pump 200, depending on the pumpingmechanism of the backing pump 200, the backing pump 200 may restrict therapid evacuation of the chamber, as the backing pump 200 may not be ableto pump the fluid exhaust from the booster pump 100 sufficientlyquickly. In order to enable at least some of the gas pumped from thechamber to by-pass the backing pump 200, as shown in FIG. 6 an externalby-pass conduit 250 may be provided in fluid communication with theexhaust 112 of the booster pump 100 and the exhaust 221 of the backingpump 200. The by-pass conduit 250 preferably includes a by-pass valve252 for opening the conduit 250 at high exhaust pressures from thebooster pump 100 to enable “excess” fluid exhaust from booster pump 100to by-pass the backing pump 200.

With reference now to FIG. 7, in order to prevent overheating of, say,the booster pump 100 during pump down of a chamber attached to the inletthereof, a the pump 100 may be provided with a temperature sensor 300located, for example, within the housing of the pump 100, for outputtingto a controller 302 a signal indicative of the current temperaturewithin the housing of the pump 100. In response to the received signal,the controller 302 can issue a command to the motor 134 of the pump 100to adjust the speed of rotation of the shaft 130. By reducing the speedof the pump, the temperature within the housing of the pump 100 can bereduced. As an alternative, or in addition, to the control of the speedof the pump in dependence on the temperature of the pump, the speed ofthe pump may also be controlled in dependence on the pressure of gasbeing conveyed to the inlet 110 of the pump using a pressure sensor 304located proximate the inlet of the pump.

In summary, two vacuum pumping arrangements are described for evacuatinga load lock chamber. In the first arrangement, a single pump comprises amulti-stage molecular drag stage and a multi-stage centrifugalcompressor mechanism exhausting pumped fluid at atmospheric pressure. Inthe second arrangement, a booster pump is provided in series with abacking pump. The booster pump is similar to the pump of the firstarrangement, but with a reduced number of compressor mechanism stages.The backing pump also comprises a multi-stage centrifugal compressormechanism exhausting pumped fluid at atmospheric pressure. Sucharrangements can reduce noise, size and vibration levels associated withconventional load lock pumps.

1. The vacuum pump arrangement according to claim 37 wherein themulti-stage centrifugal compressor mechanism comprises a housing withinwhich the drive shaft is rotatably mounted, a plurality of fixed membersdisposed within the housing and defining a plurality of interconnectedfluid chambers, the rotor elements of the compressor mechanismcomprising a plurality of impellers mounted on the drive shaft anddisposed relative to the fixed members such that each impeller deliverscompressed fluid to a respective fluid chamber, the compressor mechanismfurther comprising a bypass channel extending between two of the fluidchambers to enable fluid to pass between those chambers withoutcompression, and means for controlling the flow of fluid through thebypass channel.
 2. The vacuum pump arrangement according to claim 1wherein the control means is arranged to open the bypass channel underthe influence of a pressure difference between said two of the fluidchambers.
 3. The vacuum pump arrangement according to claim 1 whereinthe control means is arranged to open the bypass channel when thepressure in an upstream one of said two of the fluid chambers is greaterthan the pressure in a downstream one of said two of the fluid chambers.4. The vacuum pump arrangement according to claim 1 wherein said two ofthe fluid chambers are adjacent fluid chambers of the compressormechanism.
 5. The vacuum pump arrangement according to claim 4 whereinthe bypass channel passes through the fixed member located between theadjacent fluid chambers.
 6. The vacuum pump arrangement according toclaim 1 wherein the control means comprises valve means.
 7. The vacuumpump arrangement according to claim 6 wherein the valve means comprisesa valve member displaceable in use between a closed position and an openposition by pressurised fluid.
 8. The vacuum pump arrangement accordingto claim 7 wherein the valve member comprises a flap valve.
 9. Thevacuum pump arrangement according to claim 6 wherein the valve means islocated within a fluid chamber.
 10. The vacuum pump arrangementaccording to claim 1 comprising, for each fluid chamber, a respectivebypass channel extending between that fluid chamber and the adjacentdownstream fluid chamber, and means for controlling the flow of fluidthrough each bypass channel.
 11. The vacuum pump arrangement accordingto claim 1 further comprising surge control means for controlling surgewithin the multi-stage centrifugal compressor mechanism.
 12. (canceled)13. The vacuum pump arrangement according to claim 11 wherein the surgecontrol means comprises means for conveying a stream of fluid to eachfluid chamber, and means for controlling the rate of flow of the fluidstream into each fluid chamber.
 14. The vacuum pump arrangementaccording to claim 13 wherein the conveying means is arranged to conveya stream of purge gas to each fluid chamber.
 15. The vacuum pumparrangement according to claim 14 wherein the purge gas comprises air oran inert gas.
 16. The vacuum pump arrangement according to claim 13wherein the conveying means is arranged to convey a stream of compressedfluid to each fluid chamber from a downstream fluid chamber.
 17. Thevacuum pump arrangement according to claim 16 wherein the conveyingmeans comprises, for each fluid chamber, a fluid passage extendingbetween that fluid chamber and the adjacent downstream fluid chamber.18. The vacuum pump arrangement according to claim 17 wherein the fluidpassages are co-axial.
 19. The vacuum pump arrangement according toclaim 17 wherein each fluid passage passes through a respective fixedmember.
 20. The vacuum pump arrangement according to claim 16 whereinthe control means comprises valve means in fluid communication with saidconveying means.
 21. The vacuum pump arrangement according to claim 20wherein the valve means comprises a spool valve.
 22. The vacuum pumparrangement according to claim 1 wherein each fixed member comprises adisc mounted on, or integral with, a respective part of the housing. 23.The vacuum pump arrangement according to claim 1 comprising means forcooling the fixed members.
 24. The vacuum pump arrangement according toclaim 23 wherein the cooling means comprises a plurality of cooling finslocated on one side of each fixed member.
 25. The vacuum pumparrangement according to claim 23 wherein the cooling means comprisesmeans for supplying a flow of coolant to each fixed member.
 26. Thevacuum pump arrangement according to claim 1 comprising a cooling jacketextending about at least part of the multi-stage centrifugal compressormechanism.
 27. (canceled)
 28. (canceled)
 29. (canceled)
 30. (canceled)31. (canceled)
 32. A vacuum pump comprising a multi-stage centrifugalcompressor mechanism comprising a plurality of rotor elements mounted ona rotatably mounted drive shaft, and, upstream therefrom, a moleculardrag mechanism comprising at least one rotor element mounted on thedrive shaft, wherein the at least one rotor element of the moleculardrag mechanism at least partially surrounds a motor for rotating thedrive shaft.
 33. The vacuum pump according to claim 32 wherein said atleast one rotor element of the molecular drag pumping mechanismcomprises a cylinder mounted for rotary movement with the rotor elementsof the compressor mechanism.
 34. The vacuum pump according to claim 32comprising means for monitoring the temperature of the pump, and meansfor controlling the speed of rotation of the shaft in dependence on themonitored temperature.
 35. (canceled)
 36. (canceled)
 37. A vacuumpumping arrangement comprising a vacuum pump in series with a backingpump, wherein the vacuum pump comprises a multi-stage centrifugalcompressor mechanism comprising a plurality of rotor elements mounted ona rotatable mounted drive shaft, and, upstream therefrom, a moleculardrag mechanism comprising at least one rotor element mounted on thedrive shaft, wherein the at least one rotor element of the moleculardrag mechanism at least partially surrounds a motor for rotating thedrive shaft.
 38. (canceled)
 39. (canceled)
 40. The vacuum pumpingarrangement according to claim 37 comprising a bypass conduit connectedbetween an exhaust from the booster pump and an exhaust from the backingpump, and means for controlling the flow of fluid through the bypassconduit.
 41. A vacuum pump according to claim 37 wherein the moleculardrag mechanism defines a plurality of flow channels that each receivefluid from a pump inlet and exhaust pumped fluid to a common exhaustport.
 42. A vacuum pump according to claim 37 wherein the molecular dragmechanism is a multi-stage molecular drag mechanism.
 43. A vacuum pumpaccording to claim 42 wherein the stages of the drag mechanism arearranged in parallel.